1. Field of the Invention
The present invention relates to a wiring designing method and a wiring designing apparatus for electrically and mutually connecting a plurality of functional blocks of a semiconductor integrated circuit.
2. Description of the Prior Art
Integration and speed of large scaled integrated circuits (LSI) are remarkably increasing year by year. Correspondingly, the power consumptions of the circuits are increasing. Thus, the probability that a wire with a large current density melts due to electro-migration becomes high.
The electro-migration is a phenomenon that takes place in such a manner that when a current density increases in a wiring metal film or the power consumption per chip increases, the temperature of the device rises. When a high density current flows in such a high temperature situation, metal ions of the metal film migrate because of carriers. As a result, holes take place. Consequently, the current density further increases. Thus, a wire melts.
To prevent the electro-migration from taking place, the width of the wire may be increased. Alternatively, the length of the wire may be shortened by inserting an extension buffer or the like in the wiring. For example, JPA 11-97541 discloses an invention that relates to a method for designing a wiring of a semiconductor integrated circuit. The related art reference discloses a technology in which a current density of a current that flows in a wiring between functional blocks is obtained. Depending on whether the current density is less than a predetermined value, the number of wires for each branch is calculated. Then re-wiring is performed for each branch.
However, to prevent such a problem, for a net having a probability that a wire may melt due to the electro-migration, the wired result may be manually corrected. In such a case, the number of steps for correcting the wiring will increase.
Moreover, in the related art reference disclosed as JPA. 11-97541, when the current density of a current that flows in a wiring is not less than the predetermined value, the number of wires necessary for each branch is calculated. Corresponding to the calculated result, the wiring is performed once again. Thus, there is a situation that the initial wiring is not used at all. In addition, when the new wiring occupies a large area on a wiring board or a substrate, many restrictions will take place for the wiring.